Surface properties of an amphiphilic peptide hormone and of its analog: corticotropin-releasing factor and sauvagine.

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Abstract
Synthetic corticotropin (ACTH)-releasing factor [CRF] in aqueous solution exists predominantly as a random coil. At concentrations > 1 .mu.M, the peptide shows a tendency to self-aggregate with a concurrent slight increase in the apparent .alpha.-helical content as measured by the CD [circular dichroism] spectrum. The .alpha.-helix formed by this molecule is highly amphiphilic, i.e., the hydrophilic and hydrophobic regions are segregated on opposite faces of the helix. As predicted from the potential amphiphilic structure, CRF binds avidly to the surface of single bilayer egg phosphatidylcholine vesicles. This binding appears to obey a simple Langmuir isotherm with the following parameters: Kd = 1.3 .+-. 0.6 .times. 10-7 M and capacity at saturation (N) = 11.0 .+-. 1.0 mmol of peptide/mol of phospholipid. CRF also readily forms an insoluble monolayer at the air-water interface. The monolayer is composed of monomers of the hormone with molecular areas, A''0 = 22 .ANG.2 per amino acid, suggesting a compact secondary structure. Judging from the collapse pressure (19.0 .+-. 0.1 dyne/cm; 1 dyne = 10 .mu.N) of the monolayer, the amphiphilicity of CRF approximates that of plasma apolipoproteins, a class of proteins of the most pronounced amphiphilic character. The binding of CRF to the cell membrane apparently is accompanied by the induction of an .alpha.-helical secondary structure. Evidently, it is this predominantly helical form that is the biologically active form of the peptide.